German patent DE 199 38 023 for the first time discloses a hot gas engine including pistons which are movable inside one another, the range of stroke of the internal operating piston being located in the center of the range of stroke of the external piston. German patent DE 100 16 707 for the first time discloses such an engine as a free piston version.
Pressure variations of a hot gas engine may be utilized to drive diaphragms or piezo ceramics, provided the structure of the engine permits dispensing with a gear transmission for realizing one or more hot gas cycles (continuous processes). German patent DE 102 40 750, for example, describes such a gearless hot gas engine.